Memory is one type of integrated circuitry, and is used in computer systems for storing data. Integrated memory is usually fabricated in one or more arrays of individual memory cells. The memory cells may be volatile, semi-volatile, or nonvolatile. Nonvolatile memory cells can store data for extended periods of time, and in some instances can store data in the absence of power. Volatile memory dissipates and is therefore refreshed/rewritten to maintain data storage.
The memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
There is a continuing effort to produce smaller and denser integrated circuits. The smallest and simplest memory cell will likely be comprised of two electrically conductive electrodes having a programmable material received between them. Such memory cells may be referred to as cross-point memory cells.
Programmable materials suitable for utilization in cross-point memory will have two or more selectable and electrically differentiable memory states. The multiple selectable memory states can enable storing of information by an individual memory cell. The reading of the cell comprises determination of which of the memory states the programmable material is in, and the writing of information to the cell comprises placing the programmable material in a predetermined memory state. Some programmable materials retain a memory state in the absence of refresh, and thus may be incorporated into nonvolatile memory cells.
Significant interest is presently being directed toward programmable materials that utilize ions as mobile charge carriers. The programmable materials may be converted from one memory state to another by moving the mobile charge carriers therein to alter a distribution of charge density within the programmable materials. Memory devices that utilize migration of mobile charge carriers to transition from one memory state to another are sometimes referred to as Resistive Random Access Memory (RRAM) cells.
A difficulty in utilizing memory cells that simply consist of programmable material received between a pair of electrodes (i.e., cross-point memory cells) is that there can be substantial leakage of current through the devices, and such may adversely lead to errors during retrieval of stored data from a memory array. Accordingly, diodes or other select devices are commonly paired with the memory cells to assist in control of current through the memory cells. The select devices consume valuable space, and accordingly it would be desirable to develop memory cells which could perform suitably without adjacent select devices.